Fix Crystal 0.31 warnings.

src/sodium/cipher/aead/chalsa.cr

@@ -137,11 +137,11 @@ module Sodium::Cipher::Aead
         dst
       end
 
-      protected def key_size
+      protected def key_size : Int32
         KEY_SIZE
       end
 
-      protected def mac_size
+      protected def mac_size : Int32
         MAC_SIZE
       end
     end

src/sodium/cipher/chalsa.cr

@@ -105,11 +105,11 @@ module Sodium::Cipher
         end
       end
 
-      def key_size
+      def key_size : Int32
         LibSodium.crypto_stream_chacha20_ietf_keybytes.to_i32
       end
 
-      def nonce_size
+      def nonce_size : Int32
         LibSodium.crypto_stream_chacha20_ietf_noncebytes.to_i32
       end
     end

src/sodium/secure_buffer.cr

@@ -70,7 +70,7 @@ module Sodium
 
     # Returns key
     # May permanently set key to readonly depending on class usage.
-    def to_slice
+    def to_slice : Bytes
       case @state
       when State::Noaccess, State::Wiped
         readonly